How do horses see colour? What does the world look like to them?Light is detected by specialised receptors in the retina at the back of the eye. The receptors responsible for colour vision are called cones . Humans with normal colour vision possess three types of cones, each of which contains pigment that responds to a different range of colours. Because they have three separate populations of cones, roughly sensitive to red, green and blue light, normal people are termed trichromats (literally three colours ).Any change in the amount of pigment in the cones, or the wavelength of light to which it responds, can lead to deficiencies in colour vision.In contrast, most nonhuman mammals are dichromats and have only two classes of cones. Dr Evelyn Hanggi and Jerry Ingersoll of the Equine Research Foundation, Aptos, California, with the help of Terrace L Waggoner of the Eye Clinic at the Naval Hospital at Pensacola, Florida, performed a study to investigate what colours horses could distinguish. Waggoner had previously designed the Color Vision Testing Made Easy Test for children and adults and modified it for use in horses. Firstly Hanggi and Ingersoll trained the horses to chose between two cards, one of which had a circle marked on it. If the horse chose the card bearing the circle, by touching it with its nose, it received a reward. For this initial training, Hanggi used a circle composed of orange dots on a background of blue-green dots. This combination of colours was chosen because it is visible to all humans, even if they are colour-deficient.Once the horses had learned to choose the card bearing a circle, the next stage was to test different colour combinations to see if the horses could differentiate them. The choice of colours was determined by the colours that colour-blind humans have difficulty distinguishing. For example, people that are red-deficient (protanopes ) can not see a red-purple dotted circle on a dotted grey background.If a horse could not distinguish between the colours, it would be expected to choose the correct card 50% of the time purely by chance. So, to be sure that the horses’ response was not down to chance the researchers set a target of 80% correct tests before they would conclude that the horse could differentiate between the colours.None of the four horses in the study was able to distinguish a dotted brown circle on a dotted green background. Neither could they see the circle when it was made of red-purple or blue-purple dots on a dotted grey background. These findings were typical of people with red or green colour deficiencies.However, the horses could identify a circle of yellow-green dots on a grey dotted background. This shows that they were not blue colour deficient.So, it appears that, compared with humans, horses are red-green colour deficient but not blue colour deficient. It is most likely that horses see the world in a similar way to humans with red-green colour deficiencies. Red, orange, yellow and green probably all appear the same. Nonetheless, horses do just fine with limited color vision and probably use other visual cues (brightness, hue variation, depth, etc.) to function with ease in their environment.For more details see:Color vision in Horses (Equus caballus): Deficiencies identified using a pseudoisochromatic plate test.EB Hanggi, JF Ingersoll, TL Waggoner.J Comp Psychol (2007) 121(1), 65 - 72For more information of the work of the Equine Research Foundation and their learning and riding vacations and internships see: www.equineresearch.orgFor more about colour blindness in people see: http://colorvisiontesting.com